Credence.Pattern.NoListAppendInReduce (credence v0.4.4)

Performance rule: Detects acc ++ [expr] as the return value inside Enum.reduce/3 when the initial accumulator is [].

Appending to a list with ++ is O(n) — it copies the entire left-hand list on every iteration, compounding to O(n²). The auto-fix rewrites to prepend with [expr | acc] and wraps the reduce with |> Enum.reverse(), which is O(n) total.

Bad

Enum.reduce(list, [], fn item, acc ->
  acc ++ [item * 2]
end)

list |> Enum.reduce([], fn item, acc ->
  acc ++ [process(item)]
end)

Good

Enum.reduce(list, [], fn item, acc ->
  [item * 2 | acc]
end)
|> Enum.reverse()